The following background art may be regarded as useful for the understanding of the invention: U.S. Patent Publication No. 2012/120391 A1; CA Patent No. 2,811,444 A1; and, U.S. Patent Publication No. 2012/120415 A1.
The present invention relates to a coordinate measuring device. One set of coordinate measurement devices belongs to a class of instruments that measure the three-dimensional (3D) coordinates of a point by sending a laser beam to the point, where it is intercepted by a retroreflector target. The instrument finds the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter (ADM) or an interferometer. The angles are measured with an angular transducer such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest. An example of such a device is a laser tracker.
A coordinate measuring device closely related to the laser tracker is the total station. The total station, which is most often used in surveying applications, may be used to measure the coordinates of diffusely scattering or retroreflective targets. Hereinafter, the term laser tracker is used in a broad sense to include total stations.
Ordinarily a laser tracker sends a laser beam to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The apex of the cube corner, which is the common point of intersection of the three mirrors, is located at the center of the sphere. It is common practice to place the spherical surface of the SMR in contact with an object under test and then move the SMR over the surface being measured. Because of this placement of the cube corner within the sphere, the perpendicular distance from the apex of the cube corner to the surface of the object under test remains constant despite rotation of the SMR. Consequently, the 3D coordinates of a surface can be found by having a tracker follow the 3D coordinates of an SMR moved over the surface.
A gimbal mechanism within the laser tracker may be used to direct a laser beam from the tracker to the SMR. Part of the light retroreflected by the SMR enters the laser tracker and passes onto a position detector. The position of the light that hits the position detector is used by a tracker control system to adjust the rotation angles of the mechanical azimuth and zenith axes of the laser tracker to keep the laser beam centered on the SMR. In this way, the tracker is able to follow (track) the SMR.
Angular transducers such as angular encoders attached to the mechanical azimuth and zenith axes of the tracker may be used to determine the azimuth and zenith angles of the laser beam (with respect to the tracker frame of reference). The one distance measurement and two angle measurements obtained by the laser tracker are sufficient to completely specify the three-dimensional location of the SMR or other retroreflector target.
As mentioned previously, two types of distance meters may be found in laser trackers: interferometers and absolute distance meters (ADMs). In the laser tracker, an interferometer (if present) may determine the distance from a starting point to a finishing point by counting the number of increments of known length (usually the half-wavelength of the laser light) that pass as a retroreflector target is moved between the two points. If the beam is broken during the measurement, the number of counts cannot be accurately known, causing the distance information to be lost. By comparison, the ADM in a laser tracker determines the absolute distance to a retroreflector target without regard to beam breaks, which also allows switching between targets. Because of this, the ADM is said to be capable of “point-and-shoot” measurement. Initially, absolute distance meters were only able to measure stationary targets and for this reason were always used together with an interferometer. However, some modern absolute distance meters can make rapid measurements, thereby eliminating the need for an interferometer.
In its tracking mode, the laser tracker will automatically follow movements of the SMR when the SMR is in the capture range of the tracker. If the laser beam is broken, tracking will stop. The beam may be broken by any of several means: (1) an obstruction between the instrument and SMR; (2) rapid movements of the SMR that are too fast for the instrument to follow; or (3) the direction of the SMR being turned beyond the acceptance angle of the SMR. Following a beam break, in some modes of operation the beam by default remains fixed at the point of the beam break or at the last commanded position. It may be necessary for an operator to visually search for the tracking beam and place the SMR in the beam in order to lock the instrument onto the SMR and continue tracking. In another mode of operation, the beam may be automatically directed back to the SMR through the use of a camera system, as discussed hereinbelow.
Some laser trackers include one or more cameras. A camera axis may be coaxial with the measurement beam or offset from the measurement beam by a fixed distance or angle. A camera may be used to provide a wide field of view to locate retroreflectors. A modulated light source placed near the camera optical axis may illuminate retroreflectors, thereby making them easier to identify. In this case, the retroreflectors flash in phase with the illumination, whereas background objects do not. One application for such a camera is to detect multiple retroreflectors in the field of view and measure each in an automated sequence.
Some laser trackers have the ability to measure with six degrees of freedom (DOF), which may include three coordinates, such as x, y, and z, and three rotations, such as pitch, roll, and yaw. Several systems based on laser trackers are available or have been proposed for measuring six degrees of freedom.
As explained hereinabove, tracking of a retroreflector target stops when the beam is broken. In some cases, such a beam break is intentionally created by the operator—for example, to use to beam to provide a marker for the alignment of target stands or instruments. In other cases, a beam break is unintentional or unavoidable—for example, when an operator rotates the retroreflector target too much or passes the retroreflector behind an object in moving from one point to another. In cases where the beam break is unwanted, it is desirable to provide a way to conveniently steer the beam back onto the retroreflector target.
One method known in the art for conveniently steering a beam back onto a retroreflector target is to illuminate the retroreflector target with a cone of light, to view the illuminated retroreflector target with a locator camera placed in close proximity to the light source producing the cone of light, to evaluate the position of the retroreflector image on a photosensitive array contained within the locator camera, and to activate motors of the laser tracker to drive the beam of light from the tracker toward the retroreflector target. This action may be repeated if necessary to lock the light beam from the tracker onto the retroreflector target. The locking of the light beam on the retroreflector target may be recognized by the position detector receiving a relatively large amount of retroreflected light.
In one implementation of this method for steering a beam onto a retroreflector target, the locator camera system automatically finds and locks onto a nearby retroreflector target whenever the tracker loses the beam. However, this method is limited in some respects. In some cases, numerous retroreflector targets may be located within a measurement volume, and the operator may want to direct the tracker beam to a different target than the one automatically selected by the tracker following a beam break. In other cases, the operator may want the tracker beam to remain fixed in direction so that a stand or instrument may be aligned to it.
One way around this difficulty that is known in the art is to use gestures to control the behavior of a laser tracker. In one implementation using gestures, a retroreflector target is followed using one or more locator cameras and associated proximate light sources. In this implementation, the cameras may detect a particular gesture by evaluating the motion of an illuminated retroreflector target or evaluating a pattern in the power of light from the retroreflector targets. A potential disadvantage in the use of gestures is that the operator must remember the correspondence between tracker commands and gestures.
What is needed is a flexible and convenient method for acquiring retroreflector targets. In some cases, it is desirable to recapture a retroreflector target following a beam break. In other cases, it is desirable to direct a tracker beam, either broken or unbroken, to a different retroreflector target.